<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">按钮</button>
    <script>
        var btn = document.getElementById("btn");
        // function fn () {
        //     console.log(111);
        // }
        // var fn = function (){
        //     console.log(111);
        // }
        // btn.onclick = fn;
        // btn.onclick = function () {
        //     console.log(111);
        // }
    </script>
</body>
</html>